Ronaldo Nazario wasn't just stopped by injuries, he played at a time where it was much harder for players to be as fit as they are nowadays because they were not assigned nutritionists and personal trainers from a young age. The infrastructures improved so much that players being able to perform at a world class level well in their thirties isn't not even something rare anymore (Cristiano,Messi,Modric,Lewandowski,Benzema,Kroos,Ramos and many others)
He also probably wasn't the most professional player either which did not help.
Had Ronaldo been born a decade later, I think he would have been able to play at the top level for over a decade and I truly wonder how the Messi/Cristiano/R9 comparisons would have gone.